Vapor compression system

ABSTRACT

A vapor compression system having a defrost cycle is disclosed. A compressor, a condenser, an evaporator, and an expansion valve connected in a closed refrigerant loop with at least one flow control device. The at least one flow control device may control the flow of refrigerant through the closed refrigerant loop. At least one sensor monitors a predetermined parameter of the closed refrigerant loop and at least one controller receives a signal from the at least one sensor. The at least one controller controls the position of the at least one flow control device. The at least one controller initiates a defrost cycle in the closed refrigerant loop by substantially actuating at least one of the at least one flow control device to provide refrigerant flow from the compressor to the evaporator for substantially preventing formation of frost on the evaporator.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application No. 60/955,951, filed on Aug. 15, 2007, entitled SYSTEM AND METHOD FOR CONTROLLING DEFROST CYCLES, and PCT Application No. PCT/US2008/073105 filed on Aug. 14, 2008 entitled VAPOR COMRPESSION SYSTEM, both of which are incorporated by reference in their entirety into this application.

BACKGROUND

The present application relates generally to vapor compression systems, such as Heating, Ventilating, Air Conditioning, and Refrigeration (HVAC&R) systems. More specifically, the present application is directed to defrost cycles in vapor compression systems.

The formation of frost or the freezing of condensate on an evaporator in a vapor compression system may block the air passages in the evaporator and reduce the airflow through the evaporator. The reduction of airflow may adversely affect the performance of the evaporator. To remove the build-up of frost from the air passages of the evaporator, a defrost cycle may be initiated to divert discharge gas from a compressor to the evaporator. The diverted gas may raise the temperature of the evaporator and melt any frost that may have formed on the evaporator. The increased temperature and melted frost may increase evaporator performance.

SUMMARY

One embodiment relates to a vapor compression system having a compressor, a condenser, an evaporator, and an expansion valve connected in a closed refrigerant loop. The system also includes at least one flow control device to control the flow of refrigerant through the closed refrigerant loop and at least one sensor to monitor a predetermined parameter of the closed refrigerant loop. The system further includes at least one controller to receive a signal from the at least one sensor. The at least one controller actuates the at least one flow control device. The at least one controller initiates a defrost cycle in the closed refrigerant loop by actuating at least one of the at least one flow control device to provide refrigerant flow from the compressor to the evaporator for substantially preventing formation of frost on the evaporator.

Another embodiment relates to a defrost cycle for a vapor compression system with a compressor, a condenser, an evaporator, a flow control device and an expansion valve connected in a closed refrigerant loop. The flow control device is disposed between the compressor and the evaporator. The defrost cycle also includes a controller to control an extent of actuation of the flow control device based on a predetermined parameter of the closed refrigerant loop. In response to the predetermined parameter of the closed refrigerant loop being met, the activated flow control device provides refrigerant flow from the compressor to the flow control device to the evaporator, thereby raising the temperature of the evaporator and substantially eliminating the formation of frost on the evaporator.

Yet another embodiment is directed to a method for defrosting an evaporator in a vapor compression system having the steps of providing a compressor, a condenser, an evaporator, and an expansion valve connected in a closed refrigerant loop, disposing at least one flow control device in the closed refrigerant loop and monitoring a predetermined parameter of the closed refrigerant loop with at least one sensor. The method also includes the step of sending a signal from the at least one sensor to at least one controller and controlling the position of the at least one flow control device with at least one of the at least one controller based on the signal received from the at least one sensor to initiate or cancel a defrost cycle on the evaporator.

Still another embodiment is directed to a vapor compression system having a compressor, a condenser, an evaporator, and an expansion valve connected in a closed refrigerant loop. The system also includes at least one flow control device to control the flow of refrigerant through the closed refrigerant loop and at least one sensor to monitor a predetermined parameter of the evaporator. The system further includes at least one controller to receive a signal from the at least one sensor. The at least one controller controls a position of the at least one flow control device and the at least one controller initiates a defrost cycle in the closed refrigerant loop by actuating at least one of the at least one flow control device to provide refrigerant flow from the compressor to the evaporator for substantially preventing formation of frost on the evaporator.

Another embodiment is directed to a vapor compression system having at least one compressor, at least one condenser, at least one evaporator, and at least one expansion valve connected in at least one closed refrigerant loop. The system also includes at least one flow control device to control the flow of refrigerant through the at least one closed refrigerant loop and at least one sensor to monitor a predetermined parameter of the at least one evaporator. The system also includes at least one controller to receive a signal from the at least one sensor. The at least one controller control a position of the at least one flow control device and the at least one controller initiates a defrost cycle in the at least one closed refrigerant loop by actuating at least one of the at least one flow control device to provide refrigerant flow from the at least one compressor to the at least one evaporator for substantially preventing formation of frost on the at least one evaporator.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows an exemplary embodiment of a vapor compression system in a commercial environment.

FIG. 2 shows an exemplary embodiment of a vapor compression system in a residential environment.

FIG. 3 schematically illustrates an exemplary vapor compression system in an air conditioning or cooling mode of operation.

FIG. 4 schematically illustrates an exemplary vapor compression system in a heat pump or heating mode of operation.

FIG. 5 schematically illustrates an exemplary vapor compression system in accordance with the present invention.

FIG. 6 schematically illustrates an exemplary vapor compression system in accordance with the present invention.

FIG. 7 illustrates an exemplary outdoor coil for use with an exemplary vapor compression system in accordance with the present invention.

FIG. 8 schematically illustrates an exemplary vapor compression system in accordance with the present invention.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

Referring to FIG. 1, an exemplary environment for a vapor compression system 10 in a building 12 for a typical commercial setting is shown. Vapor compression system 10 may include a compressor incorporated into a rooftop unit 14 that may supply a chilled liquid that may be used to cool building 12. Vapor compression system 10 may also include a boiler 16 to supply a heated liquid that may be used to heat building 12, and an air distribution system that circulates air through building 12. The air distribution system may include an air return duct 18, an air supply duct 20 and an air handler 22. Air handler 22 may include a heat exchanger (not shown) that is connected to boiler 16 and rooftop unit 14 by conduits 24. The heat exchanger (not shown) in air handler 22 may receive either heated liquid from boiler 16 or chilled liquid from rooftop unit 14 depending on the mode of operation of vapor compression system 10. Vapor compression system 10 is shown with a separate air handler 22 on each floor of building 12. Several air handlers 22 may service more than one floor, or one air handler may service all of the floors.

Referring to FIG. 2, an exemplary environment for a vapor compression system 10 for a typical residential setting is shown. Vapor compression system 10 may include condensing unit 26 incorporated into an outdoor unit 28. Vapor compression system 10 may also include a fan 30 that draws air across coils 32 to cool refrigerant in coils 32 before the refrigerant enters residence 34 through cooling lines 36. To assist with cooling coils 32, a fin 33, or plurality of fins may be disposed to transfer heat from refrigerant tubes of coils 32. A compressor 38 may also be disposed in outdoor unit 28. Vapor compression system 10 may also include an indoor unit 40 with an evaporator 42 to further circulate refrigerant through vapor compression system 10 and provide cooling or heating to residence 34. Indoor unit 40 may be disposed in the basement 44 of residence 34 or indoor unit 40 may be disposed in any other suitable location such as the first floor in a furnace closet (not shown) of residence 34. The air distribution system of vapor compression system 10 may include a blower 46 and air ducts 48 to distribute the conditioned air (either heated or cooled) through residence 34. A thermostat (not shown) or other control may be used to control and operate vapor compression system 10.

FIG. 3 illustrates vapor compression system 10 in a cooling mode of operation, which uses tubes to carry the refrigerant between components of the system. Refrigerant flows through vapor compression system 10 within a closed refrigeration loop 50. The refrigerant may be any fluid that absorbs and extracts heat. For example, the refrigerant may be R-22, hydrofluorocarbon (HFC) based R-410a, R-407c, or R-134a, or it may be carbon dioxide (R-744a) or ammonia (R-717). Vapor compression system 10 includes control devices 52 which may enable vapor compression system 10 to cool an environment to a prescribed temperature.

Vapor compression system 10 cools an environment by cycling refrigerant within closed refrigeration loop 50 through a condenser 54, a compressor 56, an expansion device 58, and an evaporator 60. The refrigerant enters condenser 54 as a high pressure and high temperature vapor and flows through the tubes of condenser 54. A fan 62, which is driven by a motor 64, draws air across the tubes of condenser 54. Fan 62 may also push or pull air across the tubes of condenser 54. Heat from the refrigerant vapor transfers to the heated air 66 and causes the refrigerant vapor to condense into a liquid. The liquid refrigerant then flows into expansion device 58 where the refrigerant expands to become a low pressure and low temperature liquid. Expansion device 58 may be a thermal expansion valve (TXV) or any other suitable expansion device, orifice or capillary tube. After the refrigerant exits expansion device 58, some vapor refrigerant may be present in the evaporator tubes with the liquid refrigerant.

From expansion device 58, the refrigerant enters evaporator 60 and flows through the evaporator tubes. A fan 68, which is driven by a motor 70, draws air across the tubes. Heat transfer from the air to the refrigerant liquid produces cooled air 72 and causes the refrigerant liquid to boil into a vapor. Fan 68 may be replaced by a pump, which draws fluid across the tubes.

The refrigerant then flows to compressor 56 as a low pressure and low temperature vapor. Compressor 56 reduces the volume of the refrigerant vapor and increases the pressure and temperature of the vapor refrigerant. Compressor 56 may be any suitable compressor such as a screw compressor, reciprocating compressor, rotary compressor, swing link compressor, scroll compressor, or turbine compressor. Compressor 56 is driven by a motor 74, which receives power from a variable speed drive (VSD) or a direct AC or DC power source. In one embodiment, motor 74 receives fixed line voltage and frequency from an AC power source. In some applications, the motor may be driven by a variable voltage or frequency drive. The motor may be a switched reluctance (SR) motor, an induction motor, an electronically commutated permanent magnet motor (ECM), or any other suitable motor type. The refrigerant exits compressor 56 as a high temperature and high-pressure vapor that is ready to enter the condenser and begin the refrigeration cycle again.

The operation of the refrigeration cycle is governed by control devices 52. Control devices 52 may include control circuitry 76, a sensor 78, and a temperature sensor 80. Control circuitry 76 is coupled to motors 64, 70 and 74, which drive condenser fan 62, evaporator fan 68 and compressor 56, respectively. Control circuitry 76 uses information received from sensor 78 and temperature sensor 80 to determine when to operate motors 64, 70 and 74. For example, in a residential air conditioning system, sensor 78 may be a programmable twenty-four volt thermostat that provides a temperature set point to control circuitry 76. An additional sensor 80 may determine the ambient air temperature and transmit a signal to control circuitry 76 with the ambient air temperature information. Control circuitry 76 may compare the temperature value received from the sensor to the temperature set point received from the thermostat. If the temperature value from the sensor is higher than the temperature set point, control circuitry 76 may turn on motors 64, 70 and 74, to run vapor compression system 10. Additionally, control circuitry 76 may execute hardware or software control algorithms to regulate vapor compression system 10. Control circuitry 76 may include an analog to digital (A/D) converter, a microprocessor, a non-volatile memory, and an interface board. Other devices may be included in vapor compression system 10, such as additional pressure and/or temperature transducers or switches that sense temperatures and pressures of the refrigerant, the heat exchangers, the inlet, and outlet air.

FIG. 4 illustrates vapor compression system 10 operating as a cooling system in a cooling mode of operation. Vapor compression system 10 in FIG. 4 may also operate as a heat pump system in a heating mode of operation. Refrigerant flows through a reversible refrigeration/heating loop 84 in vapor compression system 10. The refrigerant may be any fluid that absorbs and extracts heat. Additionally, the heating and cooling operations are regulated by control devices 52, solenoid 104, and reversing valve 94.

Vapor compression system 10 includes an outdoor coil 86 and an indoor coil 88 that operate as heat exchangers. Coil 86 and coil 88 may function as an evaporator or a condenser depending on the operational mode. For example, when vapor compression system 10 is operating in cooling (or “AC”) mode, outdoor coil 86 functions as a condenser, releasing heat to the outdoor air, while indoor coil 88 functions as an evaporator, absorbing heat from the indoor air. When vapor compression system 10 is operating in heating mode, outdoor coil 86 functions as an evaporator, absorbing heat from the outdoor air, while indoor coil 88 functions as a condenser, releasing heat to the indoor air. A reversing valve 94 is positioned on reversible loop 84 between coil 86 and coil 88 to control the direction of refrigerant flow and to switch vapor compression system 10 between heating mode and cooling mode.

Vapor compression system 10 also includes two metering devices, or expansion device 90 and expansion device 82 for decreasing the pressure and temperature of the refrigerant before it enters the evaporator. The refrigerant enters condenser 88 as a high pressure and high temperature vapor and flows through the tubes of condenser 88. The refrigerant then flows into metering device 82 where the refrigerant expands to become a low pressure and low temperature liquid. Metering device 82 may be thermal expansion valves (TXV) or any other suitable expansion device, orifice or capillary tube. Metering device 90 and metering device 82 are used depending on the operation mode of system 10. For example, when vapor compression system 10 is operating in cooling mode, refrigerant bypasses metering device 90 and flows through metering device 82 before entering indoor coil 88, which acts as an evaporator. Similarly, when vapor compression system 10 is operating in heating mode, refrigerant bypasses metering device 82 and flows through metering device 90 before entering outdoor coil 86, which acts as an evaporator. A single metering device may be used for both heating mode and cooling mode, similar to system 10 shown in FIG. 3.

The refrigerant enters the evaporator as a liquid with a low temperature and a low pressure. In a heating mode of operation, the evaporator is outdoor coil 86 and in a cooling mode of operation, the evaporator is indoor coil 88. Vapor refrigerant may also be present in the refrigerant as a result of the expansion process that occurs in metering device 90 and metering device 82. The refrigerant flows through tubes in the evaporator and absorbs heat from the air changing the refrigerant into a vapor. In a cooling mode of operation, the indoor air passing over the tubes also may be dehumidified. The moisture from the air may be removed from the air by condensing on the outer surface of the tubes.

After exiting the evaporator, the refrigerant passes through reversing valve 94 and flows into compressor 56. Compressor 56 decreases the volume of the refrigerant vapor, and increases the temperature and the pressure of the vapor. Compressor 56 may be any suitable compressor such as a screw compressor, reciprocating compressor, rotary compressor, swing link compressor, scroll compressor, or turbine compressor.

From compressor 56, the vapor refrigerant with an increased temperature and increased pressure flows into a condenser. In cooling mode of operation, the condenser is the outdoor coil 86, and in the heating mode of operation, the condenser is the indoor coil 88. In the cooling mode of operation, a fan 62 is powered by a motor 64 and draws air over the tubes containing refrigerant vapor. Fan 62 may be replaced by or used with a pump, which draws fluid across the tubes. The heat from the refrigerant is transferred to the outdoor air, causing the refrigerant to condense into a liquid. In heating mode of operation, a fan 68 is powered by a motor 70 and draws air over the tubes containing refrigerant vapor. The heat from the refrigerant is transferred to the indoor air, causing the refrigerant to condense into a liquid.

After exiting the condenser, the refrigerant flows through the metering device (metering device 90 in heating mode, and metering device 82 in cooling mode) and returns to the evaporator (outdoor coil 86 in heating mode, and indoor coil 88 in cooling mode) where the process begins again. In both heating and cooling modes of operation, motor 74 drives compressor 56 and circulates refrigerant through the reversible refrigeration/heating loop 84. Motor 74 may receive power either directly from an AC or DC power source or from a VSD. Motor 74 may be an SR motor, an induction motor, an ECM, or any other suitable motor type.

Operation of motor 74 is controlled by control circuitry 76. Control circuitry 76 receives information from a control 78 and sensor 98, sensor 100 and sensor 80, and uses the information to control the operation of vapor compression system 10 in both cooling mode and heating mode. For example, in cooling mode, control 78 or thermostat may provide a temperature set point to control circuitry 76. Sensor 80 measures the ambient indoor air temperature and communicates the indoor air temperature level to control circuitry 76. If the air temperature is greater than the temperature set point, the vapor compression system will operate in the cooling mode of operation. Control circuitry 76 may compare the air temperature to the temperature set point and engage compressor motor 74, fan motor 64 and fan motor 70 to run the cooling system. If the air temperature is less than the temperature set point, vapor compression system 10 will operate in the heating mode of operation. Control circuitry 76 may compare the air temperature from sensor 80 to the temperature set point from control 78 and engage motor 64, motor 70 and motor 74 to run the heating system.

Control 78 may be a thermostat. Control circuitry 76 may use information received from thermostat 78 to switch vapor compression system 10 between heating mode and cooling mode. For example, if control 78 is set to cooling mode, control circuitry 76 will send a signal to a solenoid 104 to place reversing valve 94 in the air conditioning, or cooling position 106. The refrigerant may then flow through reversible loop 84 as follows. The refrigerant exits compressor 56 and is condensed in outdoor coil 86. The refrigerant is then expanded by metering device 82, and is evaporated by indoor coil 88. If control 78 is set to heating mode of operation, control circuitry 76 may send a signal to solenoid 104 to place reversing valve 94 in the heat pump position (not shown). The refrigerant may then flow through reversible loop 84 as follows. The refrigerant exits compressor 56 and is condensed in indoor coil 88. The refrigerant is then expanded by metering device 90, and is evaporated by outdoor coil 86. Control circuitry 76 may execute hardware or software control algorithms to regulate vapor compression system 10. Control circuitry 76 may include an A/D converter, a microprocessor, a non-volatile memory, and an interface board.

Control circuitry 76 also may initiate a defrost cycle when vapor compression system 10 is operating in heating mode. When the outdoor temperature approaches freezing, moisture in the outdoor air that is directed over outdoor coil 86 may condense and freeze on the coil. Sensor 98 measures the outdoor air temperature, and sensor 100 measures the temperature of outdoor coil 86. The temperature information gathered by sensor 98 and sensor 100 are provide to control circuitry 76, which determines when to initiate a defrost cycle. For example, if sensor 98 or sensor 100 provides a temperature that is less than freezing to 76 control circuitry, system 10 may initiate a defrost cycle. In defrost cycle, solenoid 104 is actuated to place reversing valve 94 to air conditioning position, or cooling position 106, and motor 64 is shut off to discontinue airflow over the tubes. Vapor Compression system 10 operates in cooling mode until the increased temperature and pressure refrigerant flowing through the outdoor coil defrosts coil 86. Once sensor 100 detects that coil 86 is defrosted, control circuitry 76 returns reversing valve 94 to heat pump position (not shown). The defrost cycle may also be set to occur at various predetermined time and temperature combinations with or without relying on sensor 98 and sensor 100.

FIG. 5 illustrates an exemplary vapor compression system 10 operating as a heat pump, similar to vapor compression system 10 shown in FIG. 4. System 10 in FIG. 5 may include sensor 120 and sensor 122. Sensor 120 may monitor vapor compression system 10 parameters, such as the temperature and/or pressure of indoor coil 88. Sensor 122 may monitor vapor compression system 10 parameters, such as temperature and/or pressure of outdoor coil 86. Depending upon the temperature and/or pressure levels in indoor coil 88 and outdoor coil 86, control devices 52 may initiate a defrost cycle to control the temperature of the evaporator (outdoor coil 86 in heating mode). The amount of hot gas received by outdoor coil 86 is regulated to maintain or increase the efficiency of system 10 during the defrost cycle.

The defrost cycle and hot gas regulation may be operated through the use of flow control devices, or valves 124, 126, 128, 130. The defrost cycle hot gas regulation may also be operated through the use of valves 124 and 126 or the use of a single three way-valve or other suitable valve or flow control device arrangement. Valve 128 and 130 are normally open (NO), and allow refrigerant to flow from compressor 56 to outdoor coil 86 in cooling mode or indoor coil 88 in heating mode. Valve 124 and valve 126 are normally closed (NC). When initiated by control devices 52, valve 128 or valve 130 may close or actuate to a position other than open or substantially open, and valve 124 or valve 126 may open or actuate to a position other than closed or substantially closed to allow a predetermined amount of hot gas from compressor 56 to bypass the coil operating as the condenser (indoor coil 88) without being cooled. The flow of hot gas may then flow directly to the evaporator (outdoor coil 86). By allowing hot gas to bypass the condenser (indoor coil 88), the temperature of the evaporator (outdoor coil 86) may be maintained at a temperature level greater than frost temperatures, and the formation of frost within the evaporator (outdoor coil 86) may be minimized. The operation of a defrost cycle altogether may be minimized since the formation of frost may be substantially prevented.

Control devices 52 may operate to maintain a system operating point within a specified envelope or predetermined operating parameters to minimize the possibility of building frost on indoor coil 88 and outdoor coil 86. In one embodiment, the operating point may maintain the system refrigerant temperature above 32 degrees Fahrenheit (deg. F.) in indoor coil 88 and outdoor coil 86 to prevent water condensate from frosting/freezing on indoor coil 88 and outdoor coil 86.

Sensor 120 and sensor 122 monitor the refrigerant temperature and/or pressure of indoor coil 88 and outdoor coil 86. In the refrigeration or air conditioning mode (not shown) of system 10, sensor 120 may monitor the temperature of the refrigerant at the inlet of indoor coil 88, which is operating as an evaporator. Alternately, sensor 120 may be disposed to monitor the refrigerant temperature at the outlet of the indoor coil 88. Once sensor 120 has obtained a temperature level for indoor coil 88, the temperature data is sent to control circuitry 76. Control circuitry 76 may be a microprocessor or any other suitable control device. Control circuitry 76 processes the temperature information gathered from indoor coil 88. If control circuitry 76 determines that the temperature in indoor coil 88 is less than a predetermined temperature threshold, for example, 32 deg. F., control circuitry 76 signals valve 124 to regulate a predetermined amount of hot gas vapor entering indoor coil 88.

The predetermined amount of hot gas entering indoor coil 88 is controlled by controlling the position of valve 124 and valve 128 to allow the predetermined amount of hot gas vapor through to indoor coil 88 and to prevent the hot gas from entering the condenser. Valve 124 regulates the hot gas flow into indoor coil 88 by allowing a predetermined amount of hot gas to bypass outdoor coil 86, which is operating as a condenser, and flow directly to indoor coil 88. Valve 124 and valve 128 may also be controlled if the refrigerant temperature in indoor coil 88 does not begin to rise to a temperature greater than the predetermined temperature threshold. For example, as the temperature in indoor coil 88 drops below 32 deg. F., valve 128 may close or actuate to a position other than substantially open, and valve 124 may open or actuate to a position other than substantially closed, to increase the amount of hot gas passing to indoor coil 88 and by passing the condenser. As the load on vapor compression system 10 increases, valve 124 may open to allow hot gas to pass to indoor coil 88 and maintain proper refrigerant conditions, such as maintaining the temperature of the refrigerant at 32 deg. F. or greater.

In the heating mode of operation, the direction of refrigerant flow is reversed from the cooling mode of operation, as shown in FIG. 5. Sensor 122 may monitor the temperature of the refrigerant at the inlet of evaporator (outdoor coil 86). Sensor 122 may be disposed to monitor the refrigerant temperature at the outlet of outdoor coil 86. Control circuitry 76 processes the temperature information from sensor 122 and outdoor coil 86, and if control circuitry 76 determines that the temperature from outdoor coil 86 is less than a predetermined temperature threshold, control circuitry 76 signals valve 124 to regulate the amount of hot gas vapor entering into outdoor coil 86. Valve 124 regulates the hot gas flow into outdoor coil 86 by allowing a predetermined amount of hot gas to bypass indoor coil 88 and flow directly to outdoor coil 86. For example, as the temperature in outdoor coil 86 drops below 32 deg. F., valve 130 may close or actuate to a position other than substantially open, and valve 126 may open or actuate to a position other than substantially closed, to increase the amount of hot gas passing to outdoor coil 86 and bypassing the condenser. As the load on vapor compression system 10 increases, valve 126 may open at least partially to allow hot gas to pass to outdoor coil 86 and maintain proper refrigerant conditions, such as maintaining the temperature of the refrigerant at 32 deg. F. or greater.

Sensors 120, 122 may also monitor the refrigerant pressure at the inlet or outlet of indoor coil 88 and outdoor coil 86. The pressure levels obtained by sensor 120 and sensor 122 are monitored by control circuitry 76, which processes the information and sends a signal, if appropriate, to valves 124, 126, 128, 130 to regulate the flow of hot gas to the coil operating as the evaporator by allowing a certain amount of the gas to bypass the coil operating as the condenser. Sensor 120 and sensor 122 may monitor both the temperature and the pressure of the refrigerant at the inlet or outlet of indoor coil 88 and outdoor coil 86. Control circuitry 76 then signals valves 124, 126, 128, 130 if appropriate, based on the temperature and pressure levels.

Valves 124, 126, 128, 130 may be actuated at a predetermined interval to obtain proper control of the flow of gasses bypassing the condenser and flowing to evaporator. Valves 124, 126, 128, 130 may operate from a variable or modular position instead of open or closed and first or second positions to control the flow of hot gas bypassing the condenser. System 10 may be equipped with two valves to obtain varying levels of capacity modulation of the system, or system 10 may have more than two valves to maintain temperature levels within system 10. System 10 may also operate with a single valve, such as a three-way valve or any other suitable flow control device arrangement to control the hot gas bypass in system 10. The flow paths from valves 124, 126, 128, 130 may be disposed at different points within system 10 than have been described in detail in the present application, so long as the discharge gas from compressor 56 is permitted to mix with the refrigerant flow prior to entering the evaporator.

In the refrigeration or air conditioning (AC) mode (not shown), control circuitry 76 uses the temperature information from sensor 120 to send a signal to open valve 124 and close valve 128, if appropriate, to allow hot gas refrigerant from compressor 56 to be mixed into the refrigerant entering indoor coil 88. Sensor 120 may also monitor refrigerant pressure levels, or both the refrigerant temperature and the refrigerant pressure. The information is sent to control circuitry 76, which would process and signal valve 124 and valve 128, as appropriate, to either open or close to allow hot gas refrigerant to be mixed into the refrigerant entering indoor coil 88.

In the heating mode of operation shown in FIG. 5, sensor 122 monitors the inlet refrigerant temperature of evaporator/outdoor coil 86 and sends a signal to control circuitry 76. Control circuitry 76 uses the information to send a signal to at least partially open valve 126 and at least partially close valve 130, if appropriate, to allow hot gas refrigerant from the compressor to be mixed into the refrigerant entering outdoor coil 86. Sensor 122 may monitor the outlet refrigerant temperature of outdoor coil 86. Control circuitry 76 may use the information to open valve 126 or actuate valve 126 to a position other than substantially closed and close valve 130 or actuate valve 130 to a position other than substantially open, to allow hot gas refrigerant to mix with the refrigerant entering outdoor coil 86.

The temperature of the refrigerant exiting and/or entering outdoor coil 86 may be maintained at, or greater than 32 deg. F. By maintaining the temperature at, or greater than, 32 deg. F., outdoor coil 86 may be maintained above freezing conditions. Sensor 122 may monitor refrigerant pressure levels, or both the refrigerant temperature and the refrigerant pressure. The information may be sent to control circuitry 76, which may process and signal valve 126 and valve 130, as appropriate, to either open or close, to the extent necessary, to allow hot gas refrigerant to be fed into the refrigerant entering outdoor coil 86.

In another embodiment, control circuitry 76 may store previous inlet (or outlet) refrigerant temperatures for outdoor coil 86 when outdoor coil 86 is operated as an evaporator. Control circuitry 76 may use this stored information to identify when the refrigerant temperature is beginning to decrease during steady outdoor ambient temperature conditions. An outdoor ambient temperature sensor may be required if outdoor ambient temperature information is not otherwise available. For example, if the outdoor conditions are approximately 40 deg. F., the refrigerant inlet temperature to outdoor coil 86 during heating mode operation may be about 30 deg. F. Control circuitry 76 may begin to feed discharge gas to outdoor coil 86 and slowly increase the flow rate of discharge gas until a 32 deg. F. refrigerant temperature is maintained at the entrance to outdoor coil 86.

As the ambient temperature decreasingly approaches 32 deg. F., control circuitry 76 may operate to delay a full defrost cycle. Control circuitry may monitor the history of the inlet refrigerant temperature at outdoor coil 86 and may recognize when the inlet refrigerant temperature begins to decrease despite a substantially constant outdoor ambient temperature. Control circuitry 76 may use an outdoor ambient temperature sensor to verify that the inlet refrigerant temperature is not decreasing as a result of a decreasing outdoor ambient temperature. If the inlet refrigerant temperature begins to decrease despite a substantially constant outdoor ambient temperature, outdoor coil 86 may be freezing and control circuitry may call for the addition of hot discharge gas to outdoor coil 86 to raise the inlet refrigerant temperature. Raising the inlet refrigerant temperature may delay the initiation of the defrost cycle. Control circuitry 76 may also recognize when it may be more efficient to initiate a full defrost cycle (instead of attempting to delay the defrost cycle) based on the future trend of the inlet refrigerant temperature and the position of the metering device controlling the hot discharge gas. Control circuitry 76 may also have a predetermined outdoor ambient temperature versus inlet refrigerant temperature correlation that may depict the flow rate of hot discharge gas and determine at what point it would be more efficient to initiate a full defrost cycle.

Referring now to FIG. 6, an exemplary vapor compression system 10 in a heating mode of operation similar to vapor compression system 10 shown in FIG. 5 is shown. Sensor 120 may monitor vapor compression system 10 parameters, such as the temperature and/or pressure of indoor coil 88. Sensor 122 may monitor vapor compression system 10 parameters, such as temperature and/or pressure of outdoor coil 86. Depending upon the temperature and/or pressure levels in indoor and outdoor coils 86, 88, control devices 52 may initiate a defrost cycle to control the temperature of the evaporator (outdoor coil 86 in heating mode).

The defrost cycle may be operated through the use of valves 124, 128, 130. Valve 128 and 130 are normally open (NO), or normally substantially open and allow refrigerant to flow from compressor 56 to outdoor coil 86 in cooling mode or indoor coil 88 in heating mode. Valve 124 is normally closed (NC). When initiated by control devices 52, valve 128 or valve 130 may close or actuate to a position other than substantially open, and valve 124 may open or actuate to a position other than substantially closed to allow a predetermined amount of hot gas from compressor 56 to bypass the evaporator (outdoor coil 86) and flow directly to the condenser (indoor coil 88). By allowing hot gas to bypass the evaporator (outdoor coil 86), the temperature of the evaporator (outdoor coil 86) may be maintained at a temperature level greater than frost temperatures, and the formation of frost within the evaporator may be reduced or minimized.

The operation of a defrost cycle may be reduced since the formation of frost may be substantially prevented. Control devices 52 may operate to maintain a system operating point within a specified envelope or predetermined operating parameters and to minimize the possibility of building frost on indoor coil 88 and outdoor coil 86. In one embodiment, the operating point may be to maintain system refrigerant temperature above 32 degrees Fahrenheit (deg. F.) in indoor coil 88 and outdoor coil 86 to prevent water condensate from frosting/freezing on indoor coil 88 and outdoor coil 86. System 10 may also operate with a single valve, such as a three-way valve (not shown), to control the flow of hot gas bypass in system 10. Valve 124 may also be connected to the suction line of compressor 56 to improve operation of system 10 during a defrost cycle of the evaporator (outdoor coil 86).

Referring to FIG. 7, sensor or sensors 138 may be installed or disposed on fins 33 of coil 32 to monitor the temperature of coil 32. If sensors 138 on fins 33 detect a temperature of 32 deg. F. when the outdoor ambient temperature is less than 32 deg. F., outdoor coil 86 may be frosting, that is, frost may be forming on coil 32. Sensors 138 communicate the temperature levels to control circuitry (not shown), which may then initiate the metering of the hot discharge gas to the coil 32 to delay any defrosting coil 32. If sensors 138 on fins 33 are disposed or located in the portion of coil 32 that may sustain the initial build up of frost, sensors 138 will detect the earliest frost conditions, initiate bypass of hot gas to outdoor coil, and delay a defrost cycle. An additional sensor or sensors may be placed in an area of coil 32 that typically frosts last. If that sensor or sensors also monitor a temperature of 32 deg. F., then a full defrost cycle may be initiated.

As shown in FIG. 8, outdoor coil 86 may defrost (or delay defrosting) one refrigerant circuit at a time if multiple refrigerant paths are present. For example, if there are ten refrigerant circuits for a one stage compressor, when a call for defrost is initiated, the discharge gas may be directed to path 134, then path 136, and so on until all ten circuits have been defrosted. For this embodiment of the defrost cycle, only one tenth of the outdoor coil is being defrosted at any given time. FIG. 8 illustrates four paths for exemplary and simplification purposes. A complex piping scheme with a variety of check valves or solenoids 132 may be needed. A control board 102 may be needed for sequencing, or the defrost cycle may be on demand by using refrigerant temperature sensors for each circuit.

While only certain features and embodiments of the invention have been illustrated and described, many modifications and changes may occur to those skilled in the art (for example, variations in sizes, dimensions, structures, shapes and proportions of the various elements, values of parameters (for example, temperatures, pressures, etc.), mounting arrangements, use of materials, colors, orientations, etc.) without materially departing from the novel teachings and advantages of the subject matter recited in the claims. The order or sequence of any process or method steps may be varied or re-sequenced according to alternative embodiments. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention. Furthermore, in an effort to provide a concise description of the exemplary embodiments, all features of an actual implementation may not have been described (that is, those unrelated to the presently contemplated best mode of carrying out the invention, or those unrelated to enabling the claimed invention). It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation specific decisions may be made. Such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure, without undue experimentation. 

1. A vapor compression system comprising: a compressor, a condenser, an evaporator, and an expansion valve connected in a closed refrigerant loop; at least one flow control device configured to control the flow of refrigerant through the closed refrigerant loop; at least one sensor configured to monitor a predetermined parameter of the closed refrigerant loop; at least one controller configured to receive a signal from the at least one sensor, the at least one controller configured to actuate the at least one flow control device; and the at least one controller configured to initiate a defrost cycle in the closed refrigerant loop by actuating at least one of the at least one flow control device to provide refrigerant flow from the compressor to the evaporator for substantially preventing formation of frost on the evaporator.
 2. The system of claim 1, wherein refrigerant flow from the compressor is provided to the condenser upstream of the evaporator.
 3. The system of claim 1, wherein the at least one controller controls the position of the at least one flow control device based on the predetermined parameter of the closed refrigerant loop.
 4. The system of claim 1, wherein the predetermined parameter of the closed refrigerant loop is the temperature of the evaporator.
 5. The system of claim 1, wherein the predetermined parameter of the closed refrigerant loop is the pressure level of the evaporator.
 6. The system of claim 4, wherein the at least one controller initiates the defrost cycle when the temperature of the evaporator reaches a predetermined temperature.
 7. The system of claim 5, wherein the controller initiates the defrost cycle when the pressure of the evaporator reaches a predetermined pressure.
 8. The system of claim 1, wherein the flow control device is a valve.
 9. The system of claim 1, wherein the vapor compression system is a heat pump system.
 10. A defrost cycle for a vapor compression system comprising: a vapor compression system comprising a compressor, a condenser, an evaporator, a flow control device and an expansion valve connected in a closed refrigerant loop, the flow control device disposed between the compressor and the evaporator; a controller configured to control an extent of actuation of the flow control device based on a predetermined parameter of the closed refrigerant loop; wherein in response to the predetermined parameter of the closed refrigerant loop being met, the activated flow control device provides refrigerant flow from the compressor to the flow control device to the evaporator, thereby raising the temperature of the evaporator and substantially eliminating the formation of frost on the evaporator.
 11. The defrost cycle of claim 10, wherein the predetermined parameter of the closed refrigerant loop is an evaporator temperature.
 12. The defrost cycle of claim 10, wherein the predetermined parameter of the closed refrigerant loop is an evaporator pressure.
 13. The defrost cycle of claim 10, wherein the flow control device is a valve.
 14. The defrost cycle of claim 10, wherein the vapor compression system is a heat pump.
 15. A method for defrosting an evaporator in a vapor compression system comprising the steps of: providing a compressor, a condenser, an evaporator, and an expansion valve connected in a closed refrigerant loop; disposing at least one flow control device in the closed refrigerant loop; monitoring a predetermined parameter of the closed refrigerant loop with at least one sensor; and sending a signal from the at least one sensor to at least one controller; controlling the position of the at least one flow control device with at least one of the at least one controller based on the signal received from the at least one sensor to initiate or cancel a defrost cycle on the evaporator.
 16. The method of claim 15, wherein the step of disposing the at least one flow control device further comprises the flow control device routing the refrigerant flow discharge from the compressor to the condenser, and the refrigerant flow discharge from the condenser to the evaporator.
 17. The method of claim 16, wherein the step of controlling the position of the flow control device further comprises the flow control device routing the refrigerant flow discharge from the compressor to the evaporator.
 18. The method of claim 15, wherein the step of monitoring a predetermined parameter of the closed refrigerant loop comprises monitoring evaporator temperature.
 19. The method of claim 15, wherein the step of monitoring a predetermined parameter of the closed refrigerant loop comprises monitoring evaporator pressure.
 20. The method of claim 15, wherein the step of disposing a flow control device in the closed refrigerant loop further comprises disposing a valve in the closed refrigerant loop.
 21. A vapor compression system comprising: a compressor, a condenser, an evaporator, and an expansion valve connected in a closed refrigerant loop; at least one flow control device configured to control the flow of refrigerant through the closed refrigerant loop; at least one sensor configured to monitor a predetermined parameter of the evaporator; at least one controller configured to receive a signal from the at least one sensor, the at least one controller configured to control a position of the at least one flow control device; and the at least one controller configured to initiate a defrost cycle in the closed refrigerant loop by actuating at least one of the at least one flow control device to provide refrigerant flow from the compressor to the evaporator for substantially preventing formation of frost on the evaporator.
 22. The system of claim 21, wherein the at least one sensor is disposed on a cooling fin of the evaporator.
 23. A vapor compression system comprising: at least one compressor, at least one condenser, at least one evaporator, and at least one expansion valve connected in at least one closed refrigerant loop; at least one flow control device configured to control the flow of refrigerant through the at least one closed refrigerant loop; at least one sensor configured to monitor a predetermined parameter of the at least one evaporator; at least one controller configured to receive a signal from the at least one sensor, the at least one controller configured to control a position of the at least one flow control device; and the at least one controller configured to initiate a defrost cycle in the at least one closed refrigerant loop by actuating at least one of the at least one flow control device to provide refrigerant flow from the at least one compressor to the at least one evaporator for substantially preventing formation of frost on the at least one evaporator.
 24. The system of claim 23, wherein one defrost cycle is initiated at a time. 